Because of its excellent physical properties, such as chemical stability and mechanical strength, alumina is used in various mechanical and electrical parts. Most of the alumina that is thus used by industry is produced by calcining aluminum hydroxide obtained by the Bayer process. However, aluminum hydroxide obtained by the Bayer process normally unavoidably contains a soda content of 0.15 to 0.80 percent by weight, as Na2O, on an alumina conversion basis. When the alumina is to be used as a raw material for electrical insulation ceramics materials for integrated circuit (IC) substrates or spark plugs and the like, the inclusion of that much soda is undesirable, since it can cause insulation defects and the like.
Various methods have therefore been proposed for removing the soda contained in the alumina. JP-A SHO 63-35573, for example, discloses a method of using aluminum hydroxide and alumina particles as raw materials to which are added a fluoride-based mineralizer and particles containing silicon oxide; JP-A HEI 10-167725 and HEI 11-49515 disclose adding alumina powder and fluoride to aluminum hydroxide and adding a silica-based compound as a soda removal agent; and JP-A HEI 7-41318 discloses a method comprising adding a fluoride-based mineralizer and α-alumina powder to aluminum hydroxide and adding a chloride-based compound as a soda removal agent. However, because in accordance with these methods a silica-based compound such as silica sand is added to remove the soda, the alumina that is obtained is contaminated by silica, degrading the sintering properties.
JP-A HEI 6-329412 discloses a two-stage method of calcinating aluminum hydroxide in the presence of a mineralizer. While with this method there is no risk that the alumina thus obtained is contaminated by silica, having to perform two calcining steps is not economical.
In recent years low soda alumina is used in the field of electronic ceramics materials such as IC substrates and IC packaging, and there is a strong need for a low-cost low soda alumina with good sintering properties that exhibits a stable shrinkage factor when sintered.
An object of the present invention is to provide a method of producing a low-cost low soda alumina that has good sintering properties, can be calcined even at a low temperature and has a stable shrinkage factor, and a low soda alumina produced by the method.
Another object of the invention is to provide a method of using the low soda alumina to produce porcelain in which the hue can be readily changed.